1. Field of the Invention
The present invention relates to a vehicle-mounted antenna substrate unit, and particularly to an antenna grounding structure.
2. Description of Related Art
Pole antennas are widely used as vehicle-mounted antennas. A pole antenna is a rod-shaped antenna that is secured to a vehicle body in an upright position. As a pole antenna protrudes from the vehicle, it serves as a component that contributes to the appearance of the vehicle. However, there are users who prefer a design in which no antenna shows in the external appearance of the vehicle.
Accordingly, in addition to pole antennas, spoiler antennas formed by providing an antenna element inside a spoiler are also widely used. A spoiler is a member attached to a vehicle body for adjusting air flow, and is also a member that contributes to the appearance of the vehicle. A spoiler may be a visor-shaped member provided at the upper part of the rear window of the vehicle, or may be a wing-shaped member provided at the rear part of the vehicle. JP 2009-177484 A and JP 2008-283609 A disclose vehicle-mounted spoiler antennas as related art of the present invention.
In general, a pole antenna secured to a vehicle body in an upright position comprises a monopole antenna having an electrical length of one quarter wavelength. In such cases, the vehicle body functions as a grounding conductor and serves as one element for exhibiting antenna performance.
Further, in a spoiler antenna, when a conductor wire having an electrical length of one quarter wavelength is used as the antenna element and a coaxial cable is used as the power feed line, the outer conductor of the coaxial cable is connected to the vehicle body at an end connected to the antenna element. In this case too, the vehicle body functions as a grounding conductor and serves as one element for exhibiting antenna performance.
However, a spoiler is formed using a non-conductive material such as plastic resin. For this reason, the grounding path from the spoiler antenna to the vehicle body may become long, resulting in it being impossible to exhibit sufficient spoiler antenna performance.